Noise, loss of synchronization of the various components in the system, and signal fade are universal problems in the transmission of digital data. Various techniques have been devised to meet these problems. The present invention discloses a system wherein first and second detection means provide a mark and space signal indication, and a ternary detection means provides an indication of a mark or space signal indeterminate condition. A technique which appears to be pertinent to the present invention is the duobinary modulation technique which provides a three level output modulation signal for purposes of increasing transmission speed.
As mentioned, digital data communication systems are quite susceptible to noise disturbances. As is known, in voice communication systems a word may still be intelligible despite some information loss; however, in data communication systems, a single bit loss may cause a loss of an entire portion of the message.
In prior art digital data communication systems, once the receiving radio modem loses synchronization, all equipments in the receiving chain lose synchronization since they are serially connected. This extends the interval of the instability (error extension) and it results in rapid system performance degradation. Obviously, this problem becomes more serious as the noise conditions become worse.
Accordingly, tactical applications for digital data communication systems, and actually digital data communication systems in general, require the maintenance of good communications despite poor propagation conditions and despite noise interference; therefore, there is need to improve the noise margins and reliability of digital data communication systems.